Group Stockpiles Weapons as U.S. Hits Militants With Airstrikes in Iraq, Refrains From Attacking Next Door

BEIRUT—The Islamic State remains largely unchallenged in its operational base in Syria, where it is drawing strength and new recruits as the world focuses on thwarting the militant group's gains in neighboring Iraq.

Despite the U.S. airstrikes in northern Iraq, the Islamic State's black flags still fly proudly over its administrative buildings and living quarters in Raqqa, northern Syria, residents said. New recruits from across the...